The Importance of Distinguishing Dissolved-Versus Oil-Droplet Phases in Assessing the Fate, Transport, and Toxic Effects of Marine Oil Pollution¹

For years, it has been known that oil released in seawater partitions into dissolved and oil-droplet phases; however, there has been little effort to discriminate between the phases in oil spill Natural Resource Damage Assessment (NRDA) programs. In 1999, portable field equipment was built for this task. By filtering 3.5 L volumes of seawater at the time of collection, method detection limits are improved and it is possible to discriminate between the phases, thereby improving understanding of oil fate and transport processes and providing more accurate toxicological assessments. First utilized in response to the M/V New Carissa oil spill at Coos Bay, Oregon, this approach proved highly successful in discriminating the phase signatures. The resulting data demonstrated that while the dissolved-phase signal appeared in places such as crab tissue and interstitial water on an otherwise clean beach, the oil-droplet phase appeared in tissues of filter-feeding Coos Bay mussels and oysters. In Port Valdez, Alaska, the portable sampler was used to assess the phase signatures in effluent from the Ballast Water Treatment Facility (BWTF) at the Alyeska Marine Terminal. The signatures were then used to reveal differential seasonal uptake in mussels at several sites within the port. During the winter, when the water column is unstratified, both dissolved- and oil-droplet phase contaminants from the BWTF diffuser can reach the upper water column, where they are transported as a surface microlayer by winds and surface currents throughout much the fjord. In the late spring, summer, and fall, when the water column is highly stratified, only the dissolved-phase components are observed in the mussels along the shoreline, as the oil droplets are preferentially trapped below the thermocline. These findings have compelled a reassessment of monitoring methods for oil spill NRDA efforts, National Pollutant Discharge Elimination System (NPDES) permitting, and general environmental monitoring.